Impact modified polyacetal compositions

ABSTRACT

The invention relates to improved polyacetal compositions composed of polyoxymethylene (component A) and a stabilized MBS core shell graft copolymer (component B) formed from a rubber-elastic core based on polybutadiene, and a hard graft shell. 
     The stabilized MBS core shell graft copolymer (component B) contains a special stabilization formulation of at least one hindered phenol, a phosphite, a sulfide, and a pH buffer system. 
     The shaped articles produced from these mixtures are particularly distinguished by an excellent low-temperature impact strength and a good heat aging performance.

This application is a divisional of application Ser. No. 08/474,030,filed Jun. 7, 1995 now U.S. Pat. No. 5,599,860, which is acontinuation-in-part of U.S. application Ser. No. 08/196,743, filed Feb.15, 1994 abandoned.

FIELD OF THE INVENTION

Polyacetal compositions, which are also referred to as polyoxymethylene(POM) compositions, are generally understood to include compositionsbased on homopolymers of formaldehyde or of cyclic oligomers offormaldehyde, for example, trioxane, the terminal groups of which areend-capped by esterification or etherification, as well as copolymers offormaldehyde or of cyclic oligomers of formaldehyde, with oxyalkylenegroups which have at least two adjacent carbon atoms in the main chain.The proportion of the comonomers can be up to 20 percent (%) by weight.

Polyacetal molding compositions have been in commercial use for manyyears. Because of their excellent mechanical properties such as highstiffness, hardness and strength, creep resistance and fatigueresistance as well as high elastic recovery and their good resistance tomany chemicals, they have applications in a wide variety of end uses,particularly in the engineering field, for example, in automotiveapplications or in household applications, for machine parts or in theelectrical or electronic industries. However for a number of potentialapplications the impact resistance and flexibility is too low.

The present invention relates to molding compositions ofpolyoxymethylene (component A) and a stabilized MBS(methacrylate-butadiene-styrene) core shell graft copolymer (componentB) in which the particles have been formed from a rubber-elastic corebased on polybutadiene and a hard graft shell. Component B contains aspecial stabilization formulation of at least one hindered phenol, aphosphite, an organic sulfide, and a pH buffer system, such as disodiumhydrogen phosphate, trisodium phosphate, a mixture of disodium hydrogenphosphate and trisodium phosphate, or a mixture of sodium hydroxide andphosphoric acid.

The shaped articles produced from these mixtures are particularlydistinguished by an excellent low-temperature impact strength and a goodheat aging performance.

BACKGROUND OF THE INVENTION

A number of methods are known from the patent literature for improvingthe toughness properties of polyacetals, by incorporating crosslinked oruncrosslinked elastomers, in some cases also grafted elastomers. Thefollowing may be mentioned as examples: POM modified with polyurethanes(German Patent No. 1,193,240), POM modified with a graft copolymerprepared on an acrylic ester/butadiene basis (German Patent No.1,964,156), POM modified with polybutadiene (U.S. Pat. No. 4,424,307) orPOM modified with a polydiene/polyalkylene oxide block copolymer (GermanPatent No. 2 408 487). However these mixtures do not show sufficientlow-temperature impact strength for many applications.

EP 156,285 and EP 181,541 describe mixtures of POM and core shellrubber-elastic graft copolymers in which the particles have been formedfrom a rubber-elastic core based on a polydiene and a hard graft shellwith improved low-temperature impact resistance. The aging behavior ofthese mixtures at elevated temperatures however is not satisfactory,which limits their application for example for automotive parts. Theobject of the present invention was, therefore, to provide a toughenedPOM composition which, compared with known systems, possesses, attemperatures down to -40° C., considerably improved toughnessproperties, and at temperatures up to 100° C. good aging propertiescombined with a satisfactory thermostability in the melt at temperaturesup to 230° C.

It has now been found that this object can be achieved, surprisingly, byemploying a specially stabilized toughening component, namely astabilized MBS core shell graft copolymer composed of a rubber-elasticpolybutadiene core, one or more hard graft shells composed of polymersmainly formed from styrene, from methyl methacrylate or from mixturesthereof (amounts below 10 % of other monomers such as ethyl acrylate, orcrosslinking or graftlinking monomers, may also be formed into thepolymers of the shell or shells), and a special stabilizationformulation. The term "hard", as known to those skilled in impactmodifier technology, means that the shell polymer is a glass at roomtemperature or above. The special stabilization formulation consists ofat least one hindered phenol, a phosphite, a sulfide, and a pH buffersystem.

In contrast, the addition of common stabilizers to a core shell graftcopolymer either did not improve the heat aging properties to asatisfactory extent or the thermostability in the melt deteriorated toan unacceptable extent.

SUMMARY OF THE INVENTION

The invention relates to an improved POM composition comprising POM(component A) and 5-50% by weight, relative to the total mixture, of astabilized core shell graft copolymer (component B) formed from arubber-elastic core based on polybutadiene and a hard graft shell.Component B, in addition to the core shell graft copolymer, contains aspecial stabilization formulation of at least one hindered phenol, aphosphite, a sulfide, and a pH buffer system, such as disodium hydrogenphosphate, trisodium phosphate, a mixture of disodium hydrogen phosphateand trisodium phosphate, or a mixture of sodium hydroxide and phosphoricacid.

Finally, the invention relates to shaped articles produced from theimproved POM composition of this type. The shaped articles produced fromthe improved POM composition are particularly distinguished by anexcellent low-temperature impact strength and a good heat agingperformance.

An essential characteristic of the improved POM composition according tothe invention is component B, which contains a special stabilizerformulation of at least one hindered phenol, a phosphite, an organicsulfide, and a pH buffer system, such as disodium hydrogen phosphate,trisodium phosphate, a mixture of disodium hydrogen phosphate andtrisodium phosphate, or a mixture of sodium hydroxide and phosphoricacid.

The improved polyacetal composition comprises polyoxymethylene(component A) and 5-50% by weight, relative to the total mixture, of:

(a) a stabilized MBS core shell graft copolymer (component B) formedfrom a rubber-elastic core comprising polybutadiene, a hard graft shell,and a stabilizer formulation composed of at least one hindered phenol, aphosphite, a sulfide and a pH buffer system, or

(b) a combination of a sulfide, a MBS core shell graft copolymercontaining one or more hindered phenols and a phosphite, and optionally,a pH buffer system, or

(c) a combination of a sulfide, a phosphite, a MBS core shell graftcopolymer containing one or more hindered phenols, and optionally, a pHbuffer system, or

(d) a combination of a sulfide, a MBS core shell graft copolymercontaining one or more hindered phenols, a pH buffer system, andoptionally, a phosphite.

DESCRIPTION OF THE INVENTION

The present invention relates to molding compositions ofpolyoxymethylene (component A) and a stabilized core shell graftcopolymer (component B) formed from a rubber-elastic core based onpolybutadiene and a hard graft shell. Component B also contains aspecial stabilization formulation. If appropriate or desired, apolymeric third component or fillers may also be present.

Component A: Polyoxymethylene

Component A, polyoxymethylene (POM), which is also referred to aspolyacetal, may be an oxymethylene homopolymer, e.g. a homopolymer offormaldehyde or trioxane, the hemi-formal groups of which have beenend-capped by acylation or etherification as disclosed, for example, inU.S. Pat. No. 3,170,896. Preferably, however, the acetal polymer is anoxymethylene copolymer prepared by copolymerizing trioxane with 0.1 to20% by weight of a cyclic ether having at least two adjacent carbonatoms. Copolymers of this type are described in U.S. Pat. No. 3,027,352of Walling et al. Such copolymers may be described as having at leastone chain containing between about 80 and about 99.9% by weightoxymethylene (--O--CH₂ --) units interspersed with between about 0.1 and20% by weight of --O--R'-units wherein R' is a divalent radicalcontaining at least two carbon atoms directly linked to each other andpositioned in the chain between the two valences with any substituent inthe R' radical being inert. Suitable comonomers are: a) cyclic ethershaving 3, 4 or 5 ring members, and b) cyclic acetals other than trioxanehaving 5-11, preferably 5, 6, 7 or 8, ring members.

The preferred copolymers are those made up of oxymethylene andoxyethylene groups, such as copolymers of trioxane with dioxolane orwith ethylene oxide, or those made up of oxymethylene and oxybutylenegroups, such as copolymers of trioxane with butanediolformal.

Also contemplated as the acetal polymer are terpolymers prepared, forexample, by reacting: a) trioxane and a cyclic ether or cyclic acetal,or b) trioxane and a cyclic ether and cyclic acetal, such as in thepreparation of the oxymethylene copolymer, with a third monomer which isa bifunctional compound such as the diglycidyl ether of ethylene glycol,diglycidyl ether and diethers of 2 mols of glycidol and 1 molformaldehyde, dioxane or trioxane, or diethers of 2 mols of glycidol and1 mol formaldehyde, dioxane or trioxane, or diethers of 2 mols ofglycidol and 1 mol of an aliphatic diol with 2 to 8 carbon atoms,preferably 2 to 4 carbon atoms, or a cycloaliphatic diol with 4 to 8carbon atoms.

Examples of suitable bi-functional compounds include the diglycidylethers of ethylene glycol, 1,4-butanediol, 1,3-butanediol,cyclobutane-1,3-diol, 1,3-propane-diol, cyclohexane-1, 4-diol and 2,4-dimethylcyclobutane-1,3diol, with butanediol diglycidyl ethers beingmost preferred. The bi-functional compound may be used for example inthe range of 0.1 to 10 percent based on the weight of the totalmonomers. The terpolymer may be prepared using the same methods known inthe art for preparing the copolymers.

The values of reduced specific viscosity (RSV values) of thepolyoxymethylene are, in general, 0.3-2.0 dl/g, preferably 0.5-1.5 dl/g(measured in butyrolactone, stabilized with 2% by weight ofdiphenylamine, at 140° C. in a concentration of 0.5 g/100 ml) and themelt flow index values (MFI 190/2.16) are in most cases between 0.02 and50 g/min. The crystallite melting point of the polyoxymethylene iswithin the range from 140 to 180° C., preferably 150-180° C.; itsdensity is 1.38-1.45 g/ml, preferably 1.40-1.43 g/ml (measured asspecified in DIN 53.479).

The POM components according to the invention can, if appropriate, alsocontain various additives, such as protective agents against thermaldegradation of the POM, nucleating agents, antistatic agents, protectiveagents against degradation by ultra-violet lights, flame-retardingagents, strip agents, lubricants, plasticizers, pigments, dyestuffs,optical brighteners, processing aids and the like, the amount of whichcan be up to 50% by weight, relative to the total improved POMcomposition.

Suitable protective agents against thermal degradation of the POM,referred to in the parent application as "stabilizers of the polyacetalphase against the effect of heat" are in particular nitrogen-containingprotective agents against thermal degradation of the POM likepolyamides, amides of polybasic carboxylic acids, amidines, hydrazines,ureas or urethanes, and alkaline earth metal salts of aliphaticmonobasic to tribasic carboxylic acids which preferably contain hydroxylgroups and have 2-20 carbon atoms, for example calcium stearate, calciumricinoleate, calcium propionate, calcium lactate and calcium citrate.

A wide variety of nitrogen-containing protective agents against thermaldegradation of the POM may be employed in the practice of thisinvention. Suitable amidine compounds (i.e., a compound containing acarbon atom doubly bonded to one nitrogen compound and singly bonded toanother) include the cyano-guanidine compounds such as cyano-guanidineitself (dicyandiamide) and other compounds containing the divalent1-cyano-3, 3 guanidine radical.

Amine substituted triazines constitute another suitable class of amidinecompounds. The preferred compounds of this class are amine substitutedderivatives of symmetrical triazines, including guanamines(2,4-diamino-sym.-triazines), melamines (2,4,6-triamino-sym.-triazine),and substituted melamines.

Other suitable nitrogen-containing protective agents against thermaldegradation of the POM include, for example, polyamides produced by theternary polymerization of caprolactam, hexamethylene diamine adipate andhexamethylene diamine sebacate, such as those marketed by the E. I.DuPont de Nemours Company of Delaware, U.S.A., under the trade nameElvamide.

The amount of nitrogen-containing stabilizer used will vary dependingupon the particular acetal polymer used and the degree of stabilitydesired.

Further protective agents against thermal degradation of the POM includeantioxidants, such as hindered phenols, for example componentscommercially available from Ciba Geigy AG under the trademarks "Irganox245", "Irganox 259", "Irganox 1010", "Irganox 1076" or "Irganox 1098".

Examples of suitable protective agent against degradation byultra-violet light, referred to in the parent application as "lightstabilizers for POM", are α-hydroxybenzophenone derivatives andbenzotriazole derivatives.

Suitable lubricants include waxes such as long chain amide waxes, longchain ester waxes or partly saponified ester waxes, oils and polyetherglycidol.

Finally, there may be added to the inventive compositions, a nucleant ornucleating agent, such as talc, other finely divided silicates, powderedsulfates or carbonates, or a terpolymer of trioxane, ethylene oxide andbutanediol diglycidyl ether.

Generally speaking, but not necessarily or mandatory, the POM-componentsof the present invention include from 0 to about 2% by weight ofhindered phenol as an antioxidant; from 0 to about 0.3% by weight of analkaline earth metal carboxylate salt; from 0 to about 1% by weight of alubricant; from 0 to about 2% by weight of a nucleant; and from 0 to 2%by weight of a nitrogen containing stabilizer compound.

More typically, but not necessarily or mandatory, the POM-componentsinclude from about 0.2 to about 1 % by weight of hindered phenol; from 0to about 0.15% by weight of an alkaline earth metal carboxylate salt;from about 0.1 to about 0.5% by weight of a lubricant; from 0 to about1% by weight of a nucleant; and from about 0.005 to about 1.5% by weightof a nitrogen containing stabilizer compound.

Component B: Specially Stabilized Core Shell Graft Copolymer

An essential characteristic of the improved POM composition according tothe invention is the content of component B, a specially stabilized coreshell graft copolymer, the amount of which is, in general between 5 and50% by weight, preferably between about 10 and about 40% by weight andparticularly between about 10 and about 30% by weight. The component Bis a specially stabilized MBS (methacrylate-butadiene-styrene) coreshell graft copolymer. The rubbery core is polybutadiene orpoly(butadiene/styrene) with a poly(methyl methacrylate) or poly(methylmethacrylate/styrene) hard shell or shells which is grafted onto therubber core by polymerizing styrene, methyl methacrylate or mixturesthereof in the presence of the rubber core. Amounts below 10% (of thestyrene, methyl methacrylate or mixtures thereof) of other monomers suchas ethyl acrylate, or crosslinking or graftlinking monomers, may also beformed into the polymers of the shell or shells. The MBS core shellgraft copolymers of the present invention are made by well knowntechniques of emulsion polymerization. The special stabilizerformulation included in component B contains at least one hinderedphenol, an organic phosphite, an organic sulfide, and a pH buffersystem, such as disodium hydrogen phosphate, trisodium phosphate, amixture of disodium hydrogen phosphate and trisodium phosphate, or amixture of sodium hydroxide and phosphoric acid.

The hindered phenols useful in the present invention include octadecyl3-(3',-5'-di-tert-butyl-4'-hydroxy phenyl)propionate, hexamethylene bis(3,5-di-tertiary- butyl-4-hydroxy hydrocinnamate),1,1,3-tris(2'-methyl-5'-tert-butyl-4'-hydroxyphenyl)butane, 2,6-di-tert-butyl cresol, ethylene bis(oxyethylene)bis(3-tert-butyl-4-hydroxy-5-methylhydrocinnamate), and mixturesthereof. As seen by this exemplification, and as known to the skilledartisan, "hindered phenol" refers to a structure having at least onetertiary alkyl group adjacent (ortho) to the phenolic hydroxyl group,preferably the tertiary alkyl group being t-butyl, and preferably theother adjacent or ortho position being a tertiary-alkyl group. Thehindered phenols may be used at levels of about 0.2 to about 1.5 weightpercent, preferably about 0.4 to about 1.0 weight percent, of componentB.

The organic phosphites of this invention include aliphatic and aromaticphosphites, such as tris(monononylphenyl) phosphite, bisnonylphenylpentaerythritol diphosphite, bis(2,4-di-tert-butylphenyl)pentaerythritoldiphosphite, tris(2,4-di-tert-butylphenyl)phosphite, tris(mixed mono-ordi-nonylphenyl)phosphite, and the like. As can be seen from the organicphosphites exemplified and as known to the skilled artisan, thephosphite will contain a --P--O-Aryl group, wherein Aryl is phenyl oralkyl-substituted phenyl. The organic phosphites may be used at levelsof about 0.1 to about 0.8 weight percent, preferably about 0.2 to about0.4 weight percent, of component B. The sulfides of this invention haveone or more of the following sulfide groups:

    --CH.sub.2 --S--R

wherein R is alkyl group of from 1 to 20 carbon atoms, and preferably 7to 12 carbon atoms. The examples include: 2,4-bis(octylthio)methyl!-o-cresol, pentaerythritol tetrakis (octylthiopropionate), trimethylolpropane tris(octyl thiopropionate) andpentaerythritol tetrakis(β-lauryl thiopropionate), and the like.Sulfides where R is an alkyl alkanoate are not contemplated in thisinvention, and examples of these sulfides include: dilaurylthiodipropionate and dimyristyl thiodipropionate. The sulfides of thisinvention may be used at levels of about 0.25 to about 2.0 weightpercent, preferably about 0.8 to about 1.6 weight percent, of componentB. Stabilizers which contain more than one of the functionalitieshindered phenol, organic sulfide, and organic phosphite are consideredin this invention.

The pH buffer system of disodium hydrogen phosphate and trisodiumphosphate is added to adjust the pH to the range of about 7 to about 11.Disodium hydrogen phosphate or trisodium phosphate may be addedsingularly to adjust the pH to the range of about 7 to about 11. Inplace of sodium salts, a pH buffer system of sodium hydroxide andphosphoric acid may be used to adjust the pH.

A surfactant may be included with the stabilizer to provide an emulsionof the stabilizer. Examples of surfactants suitable for use with thestabilizers of the invention include: sodium dodecylbenzene sulfonateand potassium oleate. The amount of the surfactant typically constitutes5 to 25% by weight of the special stabilizer formulation depending onthe specific properties of the surfactant. Besides the specialstabilizer formulation, additional stabilizers may be added to componentB, but the cost/benefit ratio decreases as more stabilizers are added,since costs will increase proportionally as more stabilizers are addedto component B.

The inventors have found that the constituents of component B may besingularly added to POM (component A) to achieve the same result as thecombination of POM (component A) and component B. By singularly is meantthat the constituents of component B need not be combined together in amixture of the constituents of Component B, but may be added separately.The order of addition of the singular components does not appear to becritical to the invention.

It should be noted that the MBS modifier requires an effectiveanti-oxidant to be present during its isolation by spray-drying orcoagulation, regardless of the end use of the MBS modifier, and by useof the specific hindered phenols of the present invention, the MBSmodifier may be isolated safely, as well as containing in an admixedform the specific hindered phenols which are specific to component B.For this reason, it is not contemplated to add all of the constituentsof component B singularly to component A.

For example, an MBS core shell graft copolymer (containing two hinderedphenols such as1,1,3-tris(2'-methyl-5'-tert-butyl-4'-hydroxyphenyl)butane and2,6-di-tert-butyl cresol and an organic phosphite, wherein the hinderedphenols and the phosphite are present at the time of isolation of theMBS graft copolymer), a sulfide, and a pH buffer system may be added asthree singular constituents to POM to get the same excellentlow-temperature impact strength and good heat aging performance resultsas combining POM (component A) and component B. Separation of theconstituents of component B and combining them singularly with POM(component A), with the exception of the required pre-combination of thehindered phenols and the MBS, is viewed as embodiments of the invention,such that an improved polyacetal composition comprising polyoxymethylene(component A) and 5-50% by weight, relative to the total mixture ofcombinations such as: a) a sulfide, a MBS core shell graft copolymercontaining one or more hindered phenols and a phosphite, and optionally,a pH buffer system, or b) a sulfide, a phosphite, a MBS core shell graftcopolymer containing one or more hindered phenols, and optionally, a pHbuffer system, or c) a sulfide, a MBS core shell graft copolymercontaining one or more hindered phenols, a pH buffer system, andoptionally, a phosphite, are contemplated by the inventors. Applicantshave found that a pH buffer system may be optional when the phosphiteand sulfide constituents of component B are singularly added to POM(component A) and pH buffering of the constituents of component B is notnecessary. Further, Applicants have found that a phosphite may beoptional when the sulfide and pH buffer constituents of component B aresingularly added to POM (component A).

Other Components

It should be understood that the compositions of the present inventioncan include in addition to the polyacetal and the stabilized core shellgraft copolymer, other additives, modifiers, fillers and ingredients, asare generally used in polyacetal molding resins.

If appropriate or desired, a polymeric third component may also bepresent, such as for example, thermoplastic polyurethanes, polyolefins,modified polyolefins, polyamides, polyacrylates, polyesters,polycarbonates or fluoropolymers, for improving other properties, forexample, friction and wear properties, processing behavior, surfaceappearance (e.g., gloss), weatherability or manufacturing cost reduction(i.e. better economic cost to make).

Furthermore, the improved POM composition according to the invention canalso contain customary fillers. The following are examples of these:filamentous reinforcing materials, such as glass fibers or carbonfibers; non-fibrous fillers, such as glass powder, graphite, carbonblack, metal powders, metal oxides, silicates, carbonates and molybdenum(IV) sulfide. These fillers can be treated with an adhesion promoter oradhesion promoter system. If used, the amount of filler is up to 50% byweight, preferably 5 to 40% by weight, relative to the total mixture.Most preferably, the mixture according to the invention does not containfillers.

In terms of the claims, the "total mixture" includes thepolyoxymethylene and additives associated with it (Component A), thestabilized MBS modifier (Component B) or stabilizers separately combinedwith the MBS modifier, and finally other additives, such as filler. Inthis total mixture, the polyoxymethylene is always the predominantcomponent, and is always more than 50% of the total mixture, preferablybetween about 90 and about 60% by weight and particularly between about90 and about 70% by weight.

Preparation

The preparation of the improved POM composition according to theinvention is effected by vigorously mixing the components at an elevatedtemperature, in general, at temperatures above the melting point ofcomponent A, that is to say at about 160 to 250° C., preferably between180 and 220° C., in units having a good mixing action, such as, forexample, mixing rolls, kneaders or preferably extruders, most preferablytwin-screw extruders. It has been found that the size and distributionof the elastomer particles in the matrix has a considerable effect onthe mechanical properties of the improved POM composition. The mixing ofthe components should, therefore, be effected in such a way that thecomponent B is distributed as homogeneously as possible in the polymermatrix, and that the particle size of the particles of the component Bin the improved POM composition according to the invention is within therange between 0.1 and 5 μm, preferably between 0.1 and 1 μm.

After melt mixing, the improved POM composition can be pelletized,chopped or ground to give granules, chips, flakes or powders.

The improved POM composition according to the invention is thermoplasticand thus accessible to all the processing procedures typical ofthermoplastic compositions.

The improved POM composition can be processed by injection molding,extrusion, melt spinning or deep-drawing to give shaped articles of anydesired kind, and is suitable as an industrial material for theproduction of semi-finished and finished components, for example tapes,rods, sheets, films, tubes and hoses and also parts of machines, forexample, casings, gearwheels, snap fittings, bearing components andcontrol elements, automobile parts especially under the hood parts suchas clips, or interior accessories such as loud speaker grills and thelike.

The following examples are presented to illustrate a few embodiments ofthe invention, but we do not intend the invention to be limited to theillustrated embodiments. All parts and percentages are by weight unlessotherwise indicated.

EXAMPLES

The following parameters and tests are used in the examples toillustrate the present invention:

MFI 190/2.16: Melt flow index as specified in DIN 53.735 at 190° C.,2.16 kg.

MVI 190/15: Melt flow volume index as specified in DIN 53.735 at 190°C., 15 kg.

Weight loss under nitrogen (N₂), 240° C.: On 1.5 g pellets in analuminum (Al) sample holder, in a thermobalance under nitrogen after onehour.

Weight loss under air, 230 ° C.: On 5.0 g pellets in an Al-pan ofdiameter of 5.5 cm in an oven under air, after 45 min. or 2 hoursrespectively.

Weight loss under air, 150° C.: On 2.5 g pellets in an Al-pan ofdiameter of 5.5 cm. in an oven under air after 64 hours.

a_(kv) : V-notched impact strength as specified in DIN 53.453 on astandardized small bar 50×6×4 mm with a v-notch of radius 1.0 mm,measured at 23° C. For heat aging at 100° C. in an oven under air the50×6×4 mm-test samples were notched before aging.

Damaging energy, Ws: As specified in DIN 53.443 on 60×60×2 mm plaques,measured at 23° C. and at 40° C.

DSC Test: Tests are made in which the time to exotherm is measured. Thetime to exotherm is the time required to achieve the maximum exothermrate as measured by differential scanning calorimetry (DSC), with a15-20 milligram sample held at 190° C. in air. In the DSC test, therelative thermal stability is clearly demonstrated by the length of timeto exotherm (exotherm providing an excellent indication of rapiddegradation.)

Yellowness index, NG: As specified in DIN 6.167/ASTM D1925, before andafter heat aging of the plaques at 100° C. or 15020 C., respectively, inan oven under air.

The pH of the emulsion was tested using a convention pH meter, such asan Orion pH meter.

Preparation of the Improved POM Compositions

As illustrated below, component A and component B and optionally othercomponents were mixed in a fluid-mixer and then fed into a twin screwextruder of the type Werner and Pfleiderer ZDSK 28, with a l/d ratio of28 and a kneading zone for an intimate mixing of the components in themelt. The melt temperature profile over the barrel of the extruder was190-220-200° C. The melt strand was cooled with water and pelletized.The pellets were dried at 80° C. under vacuum for 24 hours. The pelletswere injection molded in the customary way to the test specimens. Thecomparative examples and comparative test specimens were similarlyprepared.

EXAMPLE 1 Preparation of the MBS Polymer Latex

A stainless steel autoclave with an agitator and several entry ports wascharged with 5 parts of a diphenyl oxide sulfonate emulsifier in 2688parts of de-ionized water and 4.9 parts of sodium formaldehydesulfoxylate and brought to pH of 4.

The autoclave was evacuated and 2297 parts of butadiene, 96.8 parts ofstyrene, 12 parts of cumene hydroperoxide, and 24.6 parts of divinylbenzene were added and caused to react at 70° C. over 9 hours. Anadditional 36.9 parts of emulsifier was also added. At the end of thereaction period no further pressure drop was observed, the residualpressure was vented.

To 4000 parts of the rubber latex having approximately 48% solids, asprepared above, were added 272 parts of styrene followed by 0.544 partsof sodium formaldehyde sulfoxylate dissolved in 416 parts of de-ionizedwater and 1.088 parts cumene hydroperoxide. One hour after completion ofthe exotherm, 280 parts of methyl methacrylate, 2.816 parts of butylenedimethacrylate, 0.28 parts of sodium formaldehyde sulfoxylate dissolvedin 80 parts of de-ionized water, and 0.560 parts of cumene hydroperoxidewere added and caused to react to completion. The resulting MBS PolymerLatex had approximately 49% solids.

EXAMPLE 2 Preparation of the Stabilizer Formulation

7.83 parts of the ethylene bis(oxyethylene)bis(3-tert-butyl-4-hydroxy-5-methyl hydrocinnamate), 7.83 parts oftris(monononylphenyl) phosphite, and 23.52 parts of pentaerythritoltetrakis (β-lauryl thiopropionate) were charged to a reaction vesselwhile heating to 85° C. When the mixture began to melt (at approximately70° C.), it was vigorously agitated to yield a homogeneous melt. 43.16parts of 22.5% solution of sodium dodecyl benzene sulfonate werecharged. The emulsion was agitated for 10 minutes, and 17.1 parts ofde-ionized water were added to the emulsion. After an additional 15minutes of mixing, the stabilizer emulsion was ready for addition to theMBS polymer latex of Example 1 as described in Example 3, below. Theresulting Stabilizer Formulation had approximately 49% solids.

EXAMPLE 3 Preparation of the Stabilized MBS Polymer Emulsion (componentB)

4000 parts of MBS polymer latex as prepared in Example 1 were heated to50° C. in a reaction vessel with agitation. 114.5 parts of 2.5% solutionof sodium hydroxide and 100 parts of 2% solution of phosphoric acid wereadded to bring the pH to 7.5 to 8.0. The stabilizer emulsion prepared inExample 2 was then added. The resulting stabilized MBS polymer emulsionwas agitated for 20 minutes at 50° C., then cooled to less than 40° C.The stabilized MBS polymer was isolated by spray drying, but can beisolated by other methods such as freeze drying and coagulation. Theresulting Stabilized MBS Polymer Emulsion had approximately 48% polymersolids. After isolation, the resulting MBS Polymer contains 0.4% ofethylene bis(oxyethylene) bis(3-tert-butyl-4-hydroxy-5-methylhydrocinnamate), 0.4% of tris(monononylphenyl) phosphite, and 1.2% ofpentaerythritol tetrakis (β-lauryl thiopropionate).

COMPARATIVE EXAMPLES 4A, 4B, 4C AND 4D AND EXAMPLE 4 Comparison ofImpact Strength. Thermal Stability in the Melt and Color Stability ofVarious Stabilized MBS Modifiers in POM Composition

Below is a comparison of impact strength (notched Charpy), before andafter heat aging, thermal stability in the melt (weight loss) and colorstability (change in yellowness) in a POM composition using variousstabilized MBS modifiers.

                                      TABLE I    __________________________________________________________________________    Example       Impact    Number and            Impact                  Strength-                         Thermal Thermal Color Stability    Composition            Strength-                  Notched                         Stability in the                                 Stability in the                                         Change in    (75% POM +            Notched                  Charpy--Heat                         Melt    Melt    Yellowness    25% MBS(#))            Charpy--as                  Aged 1000 hrs                         Weight Loss                                 Weight Loss                                         Index/Heat    (% - weight            molded                  at 100° C.                         N.sub.2, 240° C., 1 h                                 air, 230° C., 2 h                                         Aging    percent)             mJ/mm.sup.2 !                   mJ/mm.sup.2 !                          %!      %!     6 h/150°0 C.    __________________________________________________________________________    Comp. Ex. 4A            38    15     0.9     19.2    37    POM + MBS (1)    Comp. Ex. 4B            19    12     0.3     15.3    21    POM + MBS (2)    Comp. Ex. 4C            27    13     0.8     13.7    23    POM + MBS (3)    Comp. Ex. 4D            50    35     1.5     54.1    19    POM + MBS (4)    Example 4            48    40     1.3     25.4    14    POM + MBS (5)    __________________________________________________________________________

COMPARATIVE EXAMPLES 4A, 4B, 4C AND 4D

MBS (1): MBS of Example I stabilized with: (a) 0.4% of 2,6-di-tert-butyl cresol, (b) 0.13% of1,1,3-tris(2'-methyl-5'tert-butyl-4'-hydroxyphenyl) butane, and (c) 0.4%of tris(mixed mono- or di-nonylphenyl)phosphite. Comparative tocomponent B.

MBS (2): MBS of Example 1 stabilized with: (a) 1.4% of octadecyl3,5-di-tert-butyl-4-hydroxyhydrocinnamate, and (b) 0.4% oftris(mono-nonylphenyl) phosphite. Comparative to component B.

MBS (3): MBS of Example 1 stabilized with: (a) 1.4% of octadecyl3,5-di-tert-butyl-4-hydroxyhydrocinnamate, and (b) 0.4% of1,1,3-tris(2'-methyl-5' tert-butyl-4'-hydroxyphenyl)butane. Comparativeto component B.

MBS (4): MBS of Example 1, with pH raised to 7.5 using 2.5% sodiumhydroxide solution and 2% phosphoric acid solution in the amounts shownin Example 3, stabilized with: (a) 0.4% ethylene bis(oxyethylene)bis(3-tert-butyl-4-hydroxy-5-methyl hydrocinnamate), (b) 0.4% oftris(mixed mono- or dinonylphenyl)phosphite, and (c) 1.2% of dilaurylthiodipropionate. Comparative to component B.

POM: Hostaform C 9021: POM-Copolymer formed from trioxane and approx. 2%by weight of ethylene oxide, available from Hoechst AG, Frankfurt amMain, Germany, under the trademark Hostaform C 9021, containing thecustomary protective agents against thermal degradation of the POM andadditives, MFI 190/2.16 is approximately 9 (Component A).

EXAMPLE 4

MBS (5): Stabilized MBS of Example 3 (component B).

POM: Hostaform C 9021 - available from Hoechst AG, Frankfurt am Main,Germany (component A).

Note: The weight percentage (25%) of the MBS Modifier in Example 4 andComparative Examples 4A, 4B, 4C and 4D, is based on the polymer solids.

COMPARATIVE EXAMPLES 5-16

Comparison of Comparative Examples 5-16 (Table II) to Examples 5-14(Table III) illustrate that the stabilized MBS Modifier (component B) ofthis invention provides improved properties in POM in comparison toother stabilized MBS modifiers.

COMPARATIVE EXAMPLES 5-11

Component A:

POM: Hostaform C 9021.

Comparative-Component B:

MBS core shell rubber elastic graft copolymer with a polybutadiene coreand a methyl methacrylate/styrene shell (MBS (1) of Comparative Example4A).

COMPARATIVE EXAMPLES 12 AND 13

Component A:

POM-Copolymer formed from trioxane and approx. 3% by weight ofbutanediolformal, available from BASF AG, Ludwigshafen, Germany, underthe trademark Ultraform N 2320, containing the customary protectiveagents against thermal degradation of the POM and additives, MF 190/2.16is approximately 9.

Comparative-Component B:

MBS core shell rubber elastic graft copolymer with a polybutadiene coreand a methyl methacrylate/styrene shell (MBS (1) of Comparative Example4A).

COMPARATIVE EXAMPLE 14

Component A:

POM-Homopolymer formed from formaldehyde, available from E. I. DuPont deNemours, Bad Homburg, Germany, under the trademark Delrin 500,containing customary protective agents against thermal degradation ofthe POM and additives, MFI 190/2.16 is approximately 10.

Comparative-Component B:

MBS core shell rubber elastic graft copolymer with a polybutadiene coreand a methyl methacrylate/styrene shell (MBS (1) of Comparative Example4A).

COMPARATIVE EXAMPLES 15 AND 16

Component A:

POM-Copolymer formed from trioxane and approx. 2% by weight of ethyleneoxide, available from Hoechst AG, Frankfurt am Main, Germany, under thetrademark Hostaform C 9021, containing the customary protective agentsagainst thermal degradation of the POM and additives, MFI 190/2.16 isapproximately 9.

Comparison-Component B:

MBS core shell rubber elastic graft copolymer with a polybutadiene coreand a methyl methacrylate/styrene shell (MBS (1) of Comparative Example4A).

COMPARATIVE EXAMPLES 8, 9, 13, 15 AND 16

By the further addition of stabilizers to the stabilized MBS modifiersin Comparative Examples 8, 9, and 13, heat aging properties were notimproved to a satisfactory extent by the addition of: (1) Irganox 1010(Pentaerythritol-tetrakis 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate!commercially available from Ciba-Geigy AG, Basel, Switzerland), or (2)Hostanox PAR 24 (Tri(2,4-di-tert.-butylphenyl) phosphite commerciallyavailable from Hoechst AG, Frankfurt am Main, Germany). In ComparativeExamples 15 and 16, the thermostability in the melt deteriorated by anunacceptable extent by adding Irganox PS 800 (Dilauryl thiodipropionatecommercially available from Ciba-Geigy AG, Basel, Switzerland).

EXAMPLES 5-11

Component A:

POM-Copolymer formed from trioxane and approx. 2% by weight of ethyleneoxide, available from Hoechst AG, Frankfurt am Main, Germany, under thetrademark Hostaform C 9021, containing the customary protective agentsagainst thermal degradation of the POM and additives, MFI 190/2.16 isapproximately 9.

Component B:

4000 gms of MBS polymer latex of Example 1, 130.7 gms of 2.5% sodiumhydroxide, 120 gms of 2% phosphoric acid and stabilizer formulation ofExample 2.

EXAMPLES 12 AND 13

POM-Copolymer formed from trioxane and approx. 3% by weight ofbutanediolformal, available from BASF AG, Ludwigshafen, Germany, underthe trademark Ultraform N 2320, containing the customary protectiveagents against thermal degradation of the POM and additives, MFI190/2.16 is approximately 9.

Component B: Same as Component B of Examples 5-11.

EXAMPLE 14

POM-Homopolymer formed from formaldehyde, available from E. I. DuPont deNemours, Bad Homburg, Germany, under the trademark Delrin 500,containing the customary protective agents against thermal degradationof the POM and additives, MFI 190/2.16 is approximately 10.

Component B: Same as Component B of Examples 5-13.

EXAMPLES 8, 9 AND 13

In these examples, stabilizers such as Irganox 1010(pentaerythritol-tetrakis 3-(3,5-di-t-butyl-4-hydroxyphenylYpropionate!commercially available from Ciba-Geigy AG, Basel, Switzerland) orHostanox PAR 24 (Tri(2,4-di-tert.-butylphenyl) phosphite commerciallyavailable from Hoechst AG, Frankfurt am Main, Germany) were added to thestabilized MBS modifiers. The addition of these stabilizers show nosignificant improvement in properties of Examples 8,9 over Example 7, orExample 13 over Example 12 (Table III). No significant improvement isseen by the use of additional stabilizers.

                                      TABLE II    __________________________________________________________________________    Results of Comparative Examples 5-16                                                    Yellow                                     Weight         ness index                                                              akv after                        MVI  Damag-                                 Damag-                                     Loss Weight                                               Yellow-                                                    after                                                         akv  heat                        190° C.,                             ing ing air, Loss air,                                               ness Index                                                    heat before                                                              aging    Comparative         15 kg                             Energy                                 Energy                                     230° C.,                                          150° C.,                                               before                                                    aging                                                         heat 1000 h at    Example Composition  cm.sup.3 /10                             23° C.                                 -40° C.                                     45 min                                          64th heat 1000 h at                                                         aging                                                              100° C.    Number  % POM/% MBS min!  j!  j!  %!   %!  aging                                                    100° C.                                                          mj/mm.sup.2                                                               mj/mm.sup.2    __________________________________________________________________________                                                              !    Comp. Example 5            Comp. A -   59.1 12.1                                 2.2 1.3  0.6  1.5  43.1 15.3 10.6            90% Hostaform C 9021            Comp B -            10% MBS(1)    Comp. Example 6            Comp. A -            80% Hostaform C 9021                        49.2 18.7                                 4.4 3.5  0.6  1.3  55.6 33.7 18.5            Comp B -            20% MBS(1)    Comp. Example 7            Comp. A -            75% Hostaform C 9021                        39.0 25.2                                 6.4 4.6  0.7  1.4  58.3 40.2 19.2            Comp B -            25% MBS(1)    Comp. Example 8            Comp. A -            74.3% Hostaform C 9021                        38.0 23.8                                 5.1 4.0  0.5  1.7  46.2 41.1 25.3            0.7% Irganox 1010            Comp B -            25% MBS(1)    Comp. Example 9            Comp. A -   38.5 24.7                                 5.3 4.1  0.6  2.1  49.4 39.3 22.6            74.5% Hostaform C 9021            0.5% Hostanox PAR 24            Comp B -            25% MBS(1)    Comp. Example            Comp. A -   32.8 27.3                                 7.6 5.4  0.7  1.6  66.8 46.4 20.4    10      70% Hostaform C 9021            Comp B -            30% MBS(1)    Comp. Example            Comp. A -   19.3 29.4                                 8.7 9.6  0.9  1.9  79.7 no   18.2    11      60% Hostaform C 9021                         broken            Comp B -            40% MBS(1)    Comp. Example            Comp. A -   36.8 25.1                                 6.2 14.2 0.5  3.7  29.9 41.3 19.8    12      75% Ultraform N 2320            Comp B -            25% MBS(1)    Comp. Example            Comp. A -   34.8 23.8                                 5.6 16.5 0.8  3.9  26.3 40.7 22.7    13      74% Ultraform N 2320            1.0% Irganox 1010            Comp B -            25% MBS(1)    Comp. Example            Comp. A - 75% Delrin 500                        50.0 21.2                                 4.8 30.9 28.9 8.3  65.4 33.6 14.4    14      Comp B 25% MBS(1)    Comp. Example            Comp. A -   38.7 24.8                                 5.5 25.4 0.4  3.8  12.4 40.8 26.8    15      74.8% Hostaform C 9021            0.2% Irganox PS 800            Comp B -            25% MBS(1)    Comp. Example            Comp. A -   38.9 24.0                                 5.3 28.4 0.4  4.1  10.5 39.4 27.3    16      74.5% Hostaform C 9021            0.5% Irganox PS 800            Comp B -            25% MBS(1)    __________________________________________________________________________

                                      TABLE III    __________________________________________________________________________    Results of Comparative Examples 5-14                                                   Yellow                                    Weight         ness index akv after                      MVI  Damag-                                Damag-                                    Loss Weight                                              Yellow-                                                   after akv  heat                      190° C.,                           ing  ing air, Loss air,                                              ness Index                                                   heat  before                                                              aging                      15 kg                           Energy                                Energy                                    230° C.,                                         150° C.,                                              before                                                   aging heat 1000 h at    Example            cm.sup.3 /10                           23° C.                                -40° C.                                    45 min                                         64th heat 1000 h at                                                         aging                                                              100° C.    Number          Composition min!  j!   j!  %!   %!  aging                                                   100° C.                                                          mj/mm.sup.2                                                               mj/mm.sup.2    __________________________________________________________________________                                                              !    Example 5          Comp. A -   67.4 11.8 2.4 2.1  0.3  1.8  3.5   16.2 14.7          90% Hostaform C 9021          Comp B -          10%*    Example 6          Comp. A -   49.4 21.0 4.7 3.2  0.3  1.4  3.9   35.2 29.4          80% Hostaform C 9021          Comp B -          20%*    Example 7          Comp. A -   40.5 26.2 5.8 3.7  0.3  1.4  4.1   50.8 40.1          75% Hostaform C 9021          Comp B -          25%*    Example 8          Comp. A -   42.0 27.3 6.2 3.0  0.3  1.8  3.9   48.6 41.2          74.3% Hostaform C 9021          0.7% Irganox 1010          Comp B -          25%*    Example 9          Comp. A -   38.2 24.3 4.8 3.3  0.4  2.3  4.5   46.3 33.1          74.5% Hostaform C 9021          0.5% Hostanox PAR 24          Comp B -          25%*    Example 10          Comp. A -   35.5 28.1 8.3 5.0  0.3  1.8  4.3   not  43.3          70% Hostaform C 9021                           broken          Comp B -          30%*    Example 11          Comp. A -   22.3 31.3 9.4 4.9  0.4  2.1  4.7   not  41.5          60% Hostaform C 9021                           broken          Comp B -          40%*    Example 12          Comp. A -   38.7 23.8 5.7 6.8  0.3  3.5  5.2   45.8 38.4          75% Ultraform N 2320          Comp B -          25%*    Example 13          Comp. A -   40.0 26.3 6.7 6.0  0.3  3.9  5.0   44.9 39.6          74% Ultraform N 2320          1.0% Irganox 1010          Comp B -          25%*    Example 14          Comp. A 75% Delrin 500                      50.6 24.5 4.9 28.7 0.9  7.8  11.2  39.4 25.7          Comp. B 25*    __________________________________________________________________________     * = Component B of Examples 5-11 (as earlier taught herein).

COMPARATIVE EXAMPLES 9 AND EXAMPLES 17-22 DSC Thermal Stability ofComponent B (Stabilized MBS Modifier): Effect of SodiumHydroxide/Phosphoric Acid (pH Buffer) Addition

Examples 17-22 illustrate that the MBS polymer latex of Example 1, whenstabilized and pH buffered by sodium hydroxide and phosphoric acid asdescribed herein, results in a thermally stable MBS Modifier, whencompared to the stabilizer packages of Comparative Examples 17-19.

                                      TABLE IV    __________________________________________________________________________                  2.5% Sodium    Example Number                  Hydroxide       DSC @ 190° C. Time    and      Stabilizer                   g/4000 g of                           2% Phosphoric                                  to Exotherm    MBS polymer latex             Package                  polymer emulsion!                           Acid  gms!                                   minutes!    __________________________________________________________________________    Comp. Ex. 17             MBS (4)                  0        0      12    MBS of Ex. 1    Comp. Ex. 18             MBS (4)                  56.8     49.7   11    MBS of Ex. 1    Comp. Ex. 19             MBS (4)                  113.5    99.1   2    MBS of Ex. 1    Example 17             Example 2                  0        0      47    MBS of Ex. 1    Example 18             Example 2                  56.8     49.7   159    MBS of Ex. 1    Example 19             Example 2                  113.5    99.1   191    MBS of Ex. 1    Example 20             13S  0        0      51    MBS of Ex. 1    Example 21             13S  56.8     49.7   280    MBS of Ex. 1    Example 22             13S  113.5    99.1   296    MBS of Ex. 1    __________________________________________________________________________     13S: Stabilizer package of Example 2 except pentaerythritol tetrakis     (lauryl thiopropionate) was replaced with     2,4bis (octylthio)methylo-cresol.

Sulfide and the Use of pH Buffer

The sulfides, especially those claimed in this invention, are goodstabilizers for MBS polymers. The use of these sulfides have adeleterious effect on thermal stability of the melt and therefore,adversely affects the processability of the resin. The Applicants' useof the pH buffers improves the thermal stability of the melt.

We claim:
 1. An improved polyacetal composition comprising more than 50weight % of the total mixture as polyoxymethylene (component A) and10-40 weight %. relative to the total mixture, of:(a) a stabilized MBScore shell graft copolymer comprising a rubber-elastic core ofpolybutadiene or poly(butadiene/styrene), a hard graft shell, and astabilizer formulation composed of one or more hindered phenols, anorganic phosphite, an organic sulfide and a pH buffer system in therange of about 7 to about 11, or (b) a combination of (i) an organicsulfide, (ii) a MBS core shell graft copolymer comprising arubber-elastic core of polybutadiene or poly(butadiene/styrene) and ahard graft shell, and containing one or more hindered phenols and anorganic phosphite, and, optionally, (iii) a pH buffer system in therange of about 7 to about 11, the organic sulfide, the MBS core-shellpolymer of (b)(ii), and the pH buffer system being added singularly, or(c) a combination of (i) an organic sulfide, (ii) a MBS core shell graftcopolymer comprising a rubber-elastic core of polybutadiene orpoly(butadiene/styrene), and a hard graft shell, and containing one ormore hindered phenols, and (iii) at least one of an organic phosphite ora pH buffer system, the organic sulfide, the MBS core-shell polymer of(c)(ii), and the at least one of an organic phosphite or a pH buffersystem being added singularly;wherein the organic sulfide contains oneor more of the group --CH₂ --S--R, wherein R is an alkyl group of from 1to 20 carbon atoms; wherein at least one hard graft shell is a polymerof methyl methacrylate or a copolymer of methyl methacrylate andstyrene, and wherein component A further contains at least oneprotective agent against thermal degradation of the polyacetal selectedfrom the group consisting of a polyamide, amide of polybasic carboxylicacid, amidine, hydrazine, urea and urethane.
 2. An improved polyacetalcomposition comprising more than 50 weight % of the total mixture aspolyoxymethylene (component A) and 10-40 weight %, relative to the totalmixture, of:(a) a stabilized MBS core shell graft copolymer comprising arubber-elastic core of polybutadiene or poly(butadiene/styrene), a hardgraft shell, and a stabilizer formulation composed of one or morehindered phenols, an organic phosphite, an organic sulfide and a pHbuffer system in the range of about 7 to about 11, or (b) a combinationof (i) an organic sulfide, (ii) a MBS core shell graft copolymercomprising a rubber-elastic core of polybutadiene orpoly(butadiene/styrene) and a hard graft shell, and containing one ormore hindered phenols and an organic phosphite, and, optionally, (iii) apH buffer system in the range of about 7 to about 11, the organicsulfide, the MBS core-shell polymer of (b)(ii), and the pH buffer systembeing added singularly, or (c) a combination of (i) an organic sulfide,(ii) a MBS core shell graft copolymer comprising a rubber-elastic coreof polybutadiene or poly(butadiene/styrene), and a hard graft shell, andcontaining one or more hindered phenols, and (iii) at least one of anorganic phosphite or a pH buffer system, the organic sulfide, the MBScore-shell polymer of (c)(ii), and the at least one of an organicphosphite or a pH buffer system being added singularly;wherein theorganic sulfide contains one or more of the group --CH₂ --S--R, whereinR is an alkyl group of from 1 to 20 carbon atoms; wherein at least onehard graft shell is a polymer of methyl methacrylate or a copolymer ofmethyl methacrylate and styrene, and wherein component A furthercontains protective agents against thermal degradation of the polyacetalconsisting of alkaline earth metal salts of aliphatic monobasic totribasic carboxylic acids having 2-20 carbon atoms.
 3. An improvedpolyacetal composition comprising more than 50 weight % of the totalmixture as polyoxymethylene (component A) and 10-40 weight %, relativeto the total mixture, of:(a) a stabilized MBS core shell graft copolymercomprising a rubber-elastic core of polybutadiene orpoly(butadiene/styrene), a hard graft shell, and a stabilizerformulation composed of one or more hindered phenols, an organicphosphite, an organic sulfide and a pH buffer system in the range ofabout 7 to about 11, or (b) a combination of (i) an organic sulfide,(ii) a MBS core shell graft copolymer comprising a rubber-elastic coreof polybutadiene or poly(butadiene/styrene) and a hard graft shell, andcontaining one or more hindered phenols and an organic phosphite, and,optionally, (iii) a pH buffer system in the range of about 7 to about11, the organic sulfide, the MBS core-shell polymer of (b)(ii), and thepH buffer system being added singularly, or (c) a combination of (i) anorganic sulfide, (ii) a MBS core shell graft copolymer comprising arubber-elastic core of polybutadiene or poly(butadiene/styrene), and ahard graft shell, and containing one or more hindered phenols, and (iii)at least one of an organic phosphite or a pH buffer system, the organicsulfide, the MBS core-shell polymer of (c)(ii), and the at least one ofan organic phosphite or a pH buffer system being addedsingularly;wherein the organic sulfide contains one or more of the group--CH₂ --S--R, wherein R is an alkyl group of from 1 to 20 carbon atoms;wherein at least one hard graft shell is a polymer of methylmethacrylate or a copolymer of methyl methacrylate and styrene, andwherein component A further contains: from 0 to 0.3% by weight of analkaline earth metal carboxylate salt; from 0 to 1% by weight of alubricant; from 0 to 2% by weight of a nucleant; and from 0 to 2% byweight of a nitrogen-containing protective agent against thermaldegradation of the polyacetal and from 0.1 to 2% by weight of hinderedphenol protective agent against thermal degradation of the polyacetal.4. The improved polyacetal composition as claimed in claim 3 wherein thenucleating agent is a talc, finely divided silicate, powdered sulfate orcarbonate, or a terpolymer of trioxane, ethylene oxide and butanedioldiglycidyl ether.